In obese adults undergoing a protein-sparing modified fast, yellow fever immunization is associated with elevated insulin and lactate levels and reduced ketone levels, while the addition of 100g glucose to the same fast is associated with a distinct metabolic profile characterized by increased glucagon without similar increases in insulin or lactate.

When a person on a low-carb diet gets vaccinated, their body normally lowers ketones and raises insulin and lactate to fuel the immune response. But if they eat sugar before the vaccine, insulin rises sharply and stops fat breakdown, so the body can't make ketones. The immune response still happens, but now the body must use glucagon to keep blood sugar up because insulin is already pulling glucose into cells. This keeps insulin and lactate from rising further and forces glucagon to step in to release more sugar from the liver.